The difference in brightness between the high point of a sunspot cycle and its low point is less than 0.1 percent of the sun's total output.

"If you run that back in time to the 17th century using sunspot records, you'll find that this amplitude variance is negligible for climate," Foukal said.

The researchers obtained accurate daily sunspot measurements dating as far back as 1874 from institutions such as the Mount Wilson Observatory near Pasadena, California, and the Royal Observatory in Greenwich, England.

Older records exist all the way back to when the telescope was invented in the 17th century, though the data become increasingly patchy with age (see a related graphic of how the Hubble Space Telescope works).

The team also derived the sun's historic strength by looking at the presence or absence of isotopes, such as beryllium 10, in ice samples from Greenland and Antarctic that reflect the past contents of Earth's atmosphere.